Immunochromatographic strip formats have become increasingly popular for quantitative and semi-quantitative assays which use visual detection schemes. This type of immunoassay involves the application of a liquid test sample suspected of containing an analyte to be detected to an application zone of an immunochromatographic test strip. The strip is comprised of a matrix material through which the fluid test medium and analyte suspended or dissolved therein can flow by capillarity from the application zone to a capture zone where a detectable signal, or the absence of such, reveals the presence of the analyte. Typically, the strip will include means for immunospecifically binding the analyte to be detected with its specific binding partner which bears the detectable label. In one such scheme, as disclosed in U.S. Pat. No. 4,446,232; the strip contains an enzyme labeled, mobile binding partner for the analyte which is in a zone of the strip downstream from the sample application zone. If analyte is present in the test sample, it will combine with its labeled binding partner to form a complex which will flow along the strip to a detection zone which contains a substrate for the enzyme label which is capable of providing a colored response in the presence of the enzyme label. The strip contains another zone in which analyte is immobilized, so that the labeled binding partner which does not combine with analyte, due to the absence of sufficient analyte in the sample, will be captured and thereby inhibited from reaching the detection zone. There have been published various modifications of this technique, all of which involve competitive specific binding systems in which the presence or absence of analyte in the test sample is determined by the detection or lack thereof of labeled binding partner in the capture zone.
An alternative to the above described immunometric assay which detects the free labeled antibody is the so called sandwich format in which the capture zone contains immobilized antibodies against an epitope of the analyte which is different than the epitope to which the labeled antibody is specific. In this format, there is formed a sandwich of the analyte between the immobilized and labeled antibodies and it is therefore an immunometric assay which detects the bound labeled antibody species. This type of immunostrip format works well in connection with the analysis of relatively low concentrations of analyte, but can be of limited utility in the analysis of fluids containing high analyte concentration. This adverse effect is caused by the presence of excessive free analyte in the sample that competes for binding with the immobilized antibody in the strip's capture band with the analyte which has become bound to the labeled antibody by interaction therewith in a portion of the strip upstream from the capture zone. This competition can result in less of the analyte/labeled antibody conjugate being captured by the capture antibody and consequently less signal being detected in the capture zone than would be in the case if there were less analyte in the test sample. A dose-response curve prepared using this type of test strip will show increasing signal with increasing analyte up to the point where the analyte concentration begins to block the interaction between the immobilized capture antibody and the analyte/labeled antibody complex. Beyond this point, increasing analyte in the test fluid results in a decrease in the signal, so that the dose-response curve indicates decreasing signal with increasing analyte. The slope of this sort of dose-response curve somewhat resembles a hook which accounts for this phenomena being known as the hook effect. Traditionally, when the hook effect is observed or suspected, the fluid sample is diluted to several dilutions to ensure the validity of the results. The high dose hook effect may not occur if sufficient labeled or capture antibody is present in the assay medium. A complete dose-response curve (low to high analyte concentration) is usually needed to verify the existence of this effect. Accordingly, sample dilution is generally carried out whenever there is reason to expect that the assay might exhibit the hook effect. It is an object of the present invention to provide a sandwich type assay method using an immunochromatographic strip whose efficacy is not affected by high analyte concentrations in the test sample and, accordingly, does not require sample dilution or reassaying of samples containing high analyte concentrations. This method involves providing a strip with at least two capture bands and optionally a collection band in which there is immobilized a binding partner for labeled antibody which will bind labeled antibody which has not formed a complex with analyte to thereby facilitate its capture in one of the capture bands. The collection band is optional since it is not needed for the assay method to work in the sandwich format. However, by using a strip which contains a collection band, each sample measurement will provide more information thereby improving the assay's sensitivity and/or precision.
In EP 0 462 376 A.sub.2 there is disclosed a procedure in which signal at the capture site and conjugate collection site of an immunochromatographic strip are detected and the analyte concentration is determined by the intensity of the signal at the capture site relative to the signal at the recovery site. Also of interest in this regard is U.S. Pat. No. 5,569,608.
In co-pending application [Ser. No. 08/900,586] there is disclosed an assay using an immunochromatographic tographic strip having multiple capture and/or collection sites in which the signal from the detectable label in the capture zone(s) and collection zone(s) is determined whereupon a final response signal is determined using an algorithm and a number of signals which are chosen in a manner suited for a particular assay to provide a value for analyte concentration.